Secondary resistance is irrelevant and isn't even tested very often except to verify the problem has been found. The resistance can be outside of normal specs and the coil can still be working fine. More importantly, there can be part of the plastic insulating housing breaking down letting the spark short internally rather than going to a spark plug. Once that happens, it will leave a carbon track behind that makes it even easier for the current to short out. That can also happen when it heats up and becomes more conductive.
A totally different problem, also possibly related to heat buildup, is an open circuit. You would have to catch that right away with the ohmmeter before it cooled down again. That is often caused by the tiny copper wire being wound tightly and pulling loose from the terminal is was soldered to internally, or there could be a hairline crack in the wire that gets bigger as the plastic housing warms up. When that gap is small enough, the spark can still jump it and the spark plug's gap. Usually that gap with its carbon track would show up as much higher than normal resistance.
If the resistance is significantly lower than it should be, the varnish has broken down that insulates each loop of wire from those next to it and some of the loops are shorted out. That reduces the maximum voltage it can produce. The number of shorted loops will change too with changes in heat, both from the engine and from internally-generated heat from the current flow. Most often this will cause a gradually decreasing maximum voltage capability that shows up as misfires at first when one spark plug requires more voltage to fire than the coil can deliver. Eventually more and more spark plugs misfire as the voltage capability of the coil continues to go down.
No one likes throwing parts at a problem but given all the testing and the things already done, I think it's time to throw a new ignition coil on it. Also, a Ford mechanic who taught a school I attended in the early 1990s stressed that if the ignition coil, pickup coil in the distributor, or the module on the side of the distributor were found to be defective, all three should be replaced. I can't remember the reason but it made sense at the time. You only have the coil left to replace out of those three.
Tuesday, December 20th, 2016 AT 10:45 AM